(1) Field of the Invention
The present invention relates to a receiving apparatus and a method of compensating for waveform degradation of received signals, and further to an apparatus and method for detecting waveform degradation, and an apparatus and method for measuring waveforms. More particularly, the invention relates to a technique suitable for use in compensation for waveform degradation an optical signal suffers due to an optical transmission line.
(2) Description of the Related Art
FIG. 20 is a block diagram showing one example of the existing optical transmitting system. In FIG. 20, an optical transmission system 100 is made up of an optical transmission apparatus 200, an optical repeater (optical amplifier) 300, and an optical receiving apparatus 400, with an optical signal sent from the optical transmitting apparatus 200 being transmitted through an optical transmission line 500 to the optical receiving apparatus 400 while being repeated/amplified properly in the optical repeater 300. Incidentally, although only one optical repeater 300 exists in the illustration of FIG. 20, naturally, two or more optical repeaters are employable, or no need therefor arises, depending on an optical signal transmission distance.
As the aforesaid optical transmission line 500, there has frequently been used a single-mode optical fiber (SMF) of a wavelength (referred to as xe2x80x9czero dispersion wavelengthxe2x80x9d) at which chromatic dispersion becomes almost zero being in a 1.3-xcexcm (micrometer) band (see a chromatic dispersion characteristic 600 in FIG. 22). The xe2x80x9cchromatic dispersionxe2x80x9d signifies the property that the propagation speed in an optical fiber varies with optical wavelength, stemming from material dispersion (see a broken line 800 in FIG. 22) or structure (waveguide) dispersion (see a chain line 900 in FIG. 22) of the optical fiber itself (that is, with respect to the zero dispersion wavelength, the long wavelength side delays while the short wavelength side advances). In other words, the xe2x80x9czero dispersion wavelengthxe2x80x9d means a wavelength at which no advance nor delay occurs while light (wavelength) propagates in an optical fiber.
The reason for the frequent use of SMF as the optical transmission line 500 is because its transmission loss is at a minimum in an optical transmission band (1.55-xcexcm band) put frequently to use for the WDM optical transmission, and long-distance transmission is feasible. However, the employment of SMF causes the waveform degradation arising from the chromatic dispersion to occur remarkably at high-speed transmission of an optical signal.
For example, in the case of long-distance transmission of a high-speed optical signal exceeding 2.5 Gb/s (gigabit/second) through the use of SMF, there occurs the phenomenon that a waveform degradation occurs in the optical signal due to the chromatic dispersion, that is, the degree of aperture of an eye pattern of the optical signal (which will be referred to hereinafter as xe2x80x9ceye aperturexe2x80x9d) becomes smaller. In this connection, the waveform degradation arising from the chromatic dispersion includes a case (see FIG. 21B) in which the eye aperture in the amplitude direction becomes smaller (waveform is rounded) as compared with the original transmission waveform (see FIG. 21A) and a case (see FIG. 21C) in which the eye aperture in the phase direction decreases (phase is compressed) with respect to the original transmission waveform.
The difference therebetween depends upon the designs (type of the optical transmission line 500, optical transmission band, chirping setting, and others) on the optical transmission system. For example, in a common optical transmission system in which optical transmission is made in an optical transmission band of 1.55 xcexcm through the use of SMF whose zero dispersion wavelength is in a 1.3-xcexcm band, if the chirping setting is made such that the rise of the waveform is at the short-wavelength side (the fall is at the long-wavelength side), then the long-wavelength side intensively receives the effect of the chromatic dispersion so that the waveform tends to be rounded. If the chirping setting is made conversely, then the adverse tendency arises.
Meanwhile, such waveform degradation stemming from the chromatic dispersion becomes more noticeable as the optical signal transmission distance (repeating distance) becomes longer to cause the deterioration of the reception sensitivity characteristic of an optical receiver 402, described later, (difficult identification/regeneration of a signal). For this reason, so far, a dispersion-shifted fiber (DSF) (see a chromatic dispersion characteristic 700 in FIG. 22) in which the zero dispersion wavelength is shifted to a 1.55-xcexcm band forming an optical transmission band has been put to use for the optical transmission line 500, thereby providing an optical transmission system 100 capable of limiting the influence of the chromatic dispersion in the optical transmission band. However, also in the case of the use of DSF, a further increase in optical signal transmission rate makes it difficult to disregard the waveform degradation arising from the chromatic dispersion as in the case of the use of SMF.
Accordingly, for whether SMF or DSF used as the optical transmission line 500, in a case in which the optical signal transmission rate becomes high to some extent, for example, a dispersion compensation fiber (DCF) 401 designed to have a chromatic dispersion characteristic contrary to the chromatic dispersion, the optical transmission line 500 suffers, has been put at the former stage of the optical receiver 402 as shown in FIG. 20 for compensating for the waveform degradation and enlarging the eye aperture.
However, in general, the degree of deterioration of the eye aperture acceptable to the optical receiver 402, i.e., the range of chromatic dispersion value permissible to the optical receiver 402, is restricted by the reception sensitivity characteristic of the optical receiver 402, and the chromatic dispersion value increases in proportion to the transmission distance (see FIG. 22); therefore, in the above-mentioned existing optical transmission system 100, there is a need to place the dispersion compensation fiber 401 having a different chromatic dispersion characteristic according to optical transmission distance (repeating distance) to show the range of chromatic dispersion value acceptable to the optical receiver 402.
Accordingly, the system sacrifices the flexibility and the needed type of dispersion compensation fiber 401 increases, which raises the cost at the system construction and the management cost after the system construction.
In addition, in the recent years, the transmission of an ultra-high-speed optical signal such as 10 Gb/s or 40 Gb/s [assuming that a 2.5-Gb/s optical signal is 16-wavelengths multiplexed (WDM: Wavelength Division Multiplex) signal, it is a 64-wavelengths multiplexed or 128-wavelengths multiplexed signal] has been realizable. In such a case, the wavelength (channel) interval becomes as extremely short as xc2xc or {fraction (1/16)} and, hence, the characteristic change resulting from the external factors such as variation in temperature of the optical transmission line 500 is not ignorable.
Thus, the improvement of the reception sensitivity characteristic of the optical receiver 402 by compensating fixedly for the chromatic dispersion through the use of the dispersion compensation fiber 401 as mentioned above encounters limitation even from the viewpoint of ultra-high speed and vary-large capacity required in the recent WDM transmission technology, and the difficulty of coping with the further increase in speed and capacity in the future is easily predictable.
Moreover, for the transmission of such an ultra-high-speed optical signal such as 10 Gb/s or 40 Gb/s, even the waveform degradation arising from the dispersion (for example, polarization mode dispersion) other than the chromatic dispersion or the nonlinear effects such as self phase modulation effect is not ignorable as the degradation factor to the reception sensitivity characteristic of the optical receiver 402. Therefore, in the case of the ultra-high-speed optical signal transmission, the simple use of the dispersion compensation fiber 401 is remote from the sufficient compensation for the waveform degradation.
The present invention has been developed with a view to eliminating the above-mentioned problems, and it is therefore an object of the invention to achieve secure compensation for the waveform degradation of a received signal arising from chromatic dispersion or the like without using a dispersion compensation fiber.
For this purpose, a receiving apparatus according to the present invention is characterized by comprising the following means:
(1) compensation characteristic variable waveform degradation compensating means capable of compensating for waveform degradation of a received signal;
(2) received waveform measuring means for measuring waveform data on the aforesaid received signal (which will be referred to hereinafter as xe2x80x9creceived waveform dataxe2x80x9d); and
(3) control means for controlling a compensation characteristic of the aforesaid waveform degradation compensating means so that a difference between frequency data on the aforesaid received signal obtained by converting the received waveform data measured by the received waveform measuring means into a frequency domain and frequency data on a reference waveform free from waveform degradation reaches a minimum.
In the receiving apparatus thus arranged according to the invention, received waveform data is measured and the compensation characteristic of the waveform degradation compensating means is controlled so that the difference between the frequency data obtained by converting the measured received waveform data into a frequency domain and the frequency data on the reference waveform shows a minimum; therefore, for example, without the use of a waveform degradation compensator having a different compensation characteristic according to transmission distance of a reception signal, one type of receiving apparatus can cope with the waveform degradation because of the enlargement of the allowable range of the waveform degradation in the receiving apparatus. In addition, the frequency data in the frequency domain of the received signal is compared with the frequency data on the reference waveform to calculate a difference therebetween, which can cope with every waveform degradation.
Accordingly, the present invention provides the following advantages.
(1) Since, for compensation for waveform degradation of a received signal, the compensation characteristic of the waveform degradation compensating means is controlled so that the difference between frequency data obtained by converting the measured received waveform data into a frequency domain and frequency data on a reference waveform reaches a minimum, the allowable range of the waveform degradation in the receiving apparatus is enlargeable. Therefore, one type of receiving apparatus can cope flexibly with waveform degradation according to a plurality of types of transmission lines (optical transmission line, and other lines) or transmission distances, thereby considerably lowering the cost at the construction of a transmission system, the management cost after the construction, and others.
(2) Since a difference between the frequency data on the received signal and the frequency data on the reference waveform is calculated so that the compensation is made to minimize the difference therebetween, even if the waveform degradation varies due to a change of the characteristic of a transmission line stemming from the external factors such as temperature variation, it is possible to follow up the variations. In addition, for example, for an optical signal received through an optical transmission line, it is possible to deal with the waveform degradation the optical signal undergoes due to the chromatic dispersion, polarization mode dispersion, nonlinear optical effect and others. This enables realizing a receiving apparatus (optical receiving apparatus) capable of sufficiently handling the transmission of an ultra-high-speed optical signal such as 10 Gb/s (gigabit per second) or 40 Gb/s.
In this case, it is also appropriate that the aforesaid received waveform measuring means is composed of an equivalent-time sampling section for equivalent-time-sampling a received signal to acquire a plurality of wave-height data on the received signal and a wave-height data recording section for recording, as received waveform data, the wave-height data acquired by the equivalent-time sampling section. This can securely record/measure the received waveform data through the equivalent-time sampling even if the received signal is a high-speed signal.
Thus, the recording/measurement of the received waveform data is surely feasible regardless of whether or not the received signal is a high-speed signal, so a receiving apparatus is realizable which is capable of implementing sufficient compensation control on a high-speed signal.
In addition, it is also appropriate that the aforesaid equivalent-time sampling section is composed of a sampling timing generating circuit for generating a sampling timing on the basis of a reference clock synchronized with the received signal to output the sampling timing while shifting a timing of output thereof periodically, and a sampling circuit for sampling the received signal in accordance with the sampling timing generated in the sampling timing generating circuit to acquire the aforesaid wave-height data. This can surely realize the waveform measurement according to the equivalent-time sampling.
In this connection, it is preferable that the sampling timing generating circuit includes a divider for dividing the reference clock down and a delay control circuit for supplying the output of the divider to the sampling circuit while delaying periodically. This can generate the sampling timing with a proper operating clock at all times in the apparatus irrespective of the rate (frequency) of the reference clock. Moreover, even in the case of the employment of a divider designed to perform the dividing down into integer (N) times, the sampling timing can be outputted at an arbitrary timing through the delay control by the delay control circuit.
Accordingly, irrespective of the rate (frequency) of the reference clock, it is possible to generate a sampling timing with an appropriate operating timing at all times in the apparatus. In addition, even with the divider capable of dividing down into integer (N) times, it is possible to output the sampling timing at an arbitrary timing through the delay control by the delay control circuit, which can realize the equivalent-time sampling using a simple construction and showing extremely high flexibility.
Furthermore, it is preferable that the aforesaid control means includes the following sections:
(1) a reference waveform data storing section for storing the frequency data on the reference waveform previously;
(2) a Fourier-transform section for performing Fourier transform of the received waveform data to obtain frequency data on the received signal;
(3) a difference calculating section for calculating a difference between the frequency data obtained by the Fourier transform section and the frequency data in the reference waveform data storing section;
(4) a compensation characteristic determining section for obtaining a compensation characteristic of the waveform degradation compensating means which minimizes the difference calculated by the difference calculating section; and
(5) a control signal generating section for generating a control signal for controlling the aforesaid waveform degradation compensating means so that the waveform degradation compensating means has the compensation characteristic obtained by the compensation characteristic determining section.
This can univocally determine the optimum compensation characteristic of the waveform degradation compensating means which minimizes the difference of the frequency data calculated by the difference calculating section, and hence, a sweeping operation or the like for obtaining the optimum compensation characteristic becomes unnecessary, for example, as in the case in which a received signal quantity is monitored to control the compensation characteristic of the waveform degradation compensating means so that the monitored quantity reaches a predetermined quantity.
Accordingly, fast and certain compensation control becomes feasible.
If the aforesaid waveform degradation compensating means is constructed with an equalization amplifier having a variable frequency/phase characteristic as the aforesaid compensation characteristic, then it is possible to surely realize the waveform degradation compensation of a received signal (enlargement of the waveform degradation allowable range of the receiving apparatus), which contributes greatly to the realization of this receiving apparatus.
In this case, for example, if the equalization amplifier is made up of a plurality of band-pass filters having different pass bands with respect to the received signal, a plurality of phase shifters and a plurality of gain-variable amplifiers, then it is possible to provide an equalization amplifier capable of adjusting the frequency characteristic (amplitude and phase) at every pass band, thus realizing the waveform degradation compensation based on the equalization amplifier with high accuracy.
This can contribute greatly to the realization of this receiving apparatus.
In addition, in a case in which the received signal is a signal which is received through an optical transmission line and undergoes waveform degradation based on a chromatic dispersion characteristic of the optical transmission line as the aforesaid waveform degradation, if the waveform degradation compensating means is constructed with a variable dispersion compensator having a variable dispersion compensation characteristic as the aforesaid compensation characteristic while the aforesaid control means is designed to control the dispersion compensation characteristic of this variable dispersion compensator, it is possible to compensate for the waveform degradation due to the chromatic dispersion characteristic for enlarging the waveform degradation allowable range of the receiving apparatus (optical receiving apparatus) without using a different chromatic dispersion compensating fiber according to length of the optical transmission line (that is, transmission distance on a signal to be received).
Accordingly, one type of receiving apparatus can deal flexibly with the waveform degradation based on the dispersion characteristic according to a plurality of optical transmission lines or transmission distances without using different chromatic dispersion compensating fibers according to types of optical transmission lines or lengths thereof, which can realize a low-priced receiving apparatus and considerably reduce the cost at the construction of an optical transmission system and the management cost after the construction thereof.
Furthermore, a receiving apparatus according to the present invention is characterized by comprising the following components:
(1) a demultiplexing section for receiving a wavelength-multiplexed optical signal (which will be referred to as hereinafter as a xe2x80x9cWDM signalxe2x80x9d) comprising a plurality of types of wavelengths multiplexed, through an optical transmission line to perform demultiplexing according to wavelength;
(2) compensation characteristic variable type waveform degradation compensating means capable of compensating for waveform degradation the WDM signal is subjected to due to a dispersion characteristic of the optical transmission line;
(3) received waveform degradation detecting means provided with respect to at least one optical signal with a specified wavelength, of the demultiplexed optical signals from the demultiplexing section for detecting a difference between frequency data in a frequency domain of a received signal after photoelectric conversion of the optical signal and frequency data on a reference waveform free from waveform degradation; and
(4) control means for controlling a compensation characteristic of the waveform degradation compensating means so that the difference on the specified wavelength, obtained by the received waveform degradation detecting means, reaches a minimum.
In the receiving apparatus thus constructed according to the present invention, the difference of at least one received signal with a specified wavelength in frequency data from a reference waveform which does not suffer waveform degradation is obtained through calculation to control the compensation characteristic of the waveform degradation compensating means on the basis of the difference therebetween for compensating for the waveform degradation of the WDM signal; therefore, without employing a dispersion compensation fiber having a different dispersion compensation characteristic according to WDM signal transmission distance, one receiving apparatus can cope with the waveform degradation owing to the enlargement of the allowable range of the waveform degradation of the received signal.
In other words, since the difference (waveform degradation) detecting system undertakes only one wavelength at the smallest, not only the advantages and effects similar to those mentioned above are attainable but also the size reduction of the receiving apparatus is achievable.
In addition, since the compensation control is implemented on the basis of the difference in frequency data calculated, it can cope with the waveform degradation the optical signal undergoes due to not only the chromatic dispersion but also the polarization mode dispersion or nonlinear optical effects. In consequence, it can deal sufficiently with the transmission of an ultra-high-speed optical signal, such as 10Gb/s (gigabit per second) or 40 Gb/s.
In this case, it is also appropriate that the aforesaid waveform degradation compensating means is constructed as a dispersion characteristic variable type variable optical dispersion compensator placed at the former stage of the demultiplexing section, while the control means is constructed as a batch compensation control section made to batch-control a dispersion characteristic of the variable optical dispersion compensator in common with respect to the aforesaid wavelengths.
With this configuration, since the compensation control on a multi-wavelength optical signal can be executed in batch in common with respect to the wavelengths, the scale reduction of the control system is achievable, which leads to the size reduction of the receiving apparatus.
In particular, in this case, since the compensation takes place at the stage of an optical signal, as compared with the compensation at the stage of an electric signal after the photoelectric conversion, the received signal waveform degradation allowable range is further enlargeable, thus lengthening the optical transmission distance one type of receiving apparatus can deal with. This further reduce the cost at the construction of the optical transmission system and the management cost after the construction thereof.
Still additionally, it is also appropriate that the aforesaid waveform degradation compensating means is constructed with a plurality of dispersion characteristic variable type variable optical dispersion compensators each provided for each of optical signals before photoelectric conversion at the latter stage of the aforesaid demultiplexing section while the control means is constructed as an individual compensation control section for calculating differences on all wavelengths on the basis of the aforesaid difference with respect to the aforesaid specified wavelength to control dispersion compensation characteristics of the variable optical dispersion compensators individually so that each of the differences reaches a minimum.
With this arrangement, since the compensation control is implemented with respect to each of the wavelengths, as compared with the batch compensation control, it is possible to realize the compensation with higher accuracy. Moreover, also in this case, the compensation is made at the stage of the optical signal to enlarge the received signal waveform degradation allowable range, thereby further lengthening the optical transmission distance one type of receiving apparatus can deal with.
Furthermore, it is also appropriate that the aforesaid received waveform degradation detecting means is provided with respect to each of the wavelengths and the aforesaid waveform degradation compensating means is composed of a dispersion characteristic variable type variable optical dispersion compensator placed at the former stage of the aforesaid demultiplexing section and a plurality of frequency/phase characteristic variable type equalization amplifiers placed with respect to each of electric signals after the photoelectric conversion of the demultiplexed optical signals from the aforesaid demultiplexing section, while the aforesaid control means is composed of variable optical dispersion compensators each for controlling a frequency/phase characteristic of the corresponding equalization amplifier according to wavelength so that the difference detected by the corresponding received waveform degradation detecting means reaches a minimum and a batch compensation control section for batch-controlling the dispersion characteristics of the variable optical dispersion compensators on the basis of the differences on the aforesaid specified wavelength in common with respect to the wavelengths.
Since this configuration performs the compensation at two stages, that is, the batch compensation by the variable optical dispersion compensator at the stage of the optical signal (WDM signal) in common with respect to the respective wavelengths and the compensation by the equalization amplifier at the stage of the electric signal with respect to each wavelength after the photoelectric conversion, it is possible to enhance the waveform degradation compensation ability as a whole, and further to additionally enlarge the received signal waveform degradation allowable range.
In addition, in this case, since the compensation at the stage of the optical signal is first performed so that the compensation by the equalization amplifier is made in a state where the waveform degradation is compensated to some extent, it is possible to lighten the compensation ability (frequency/phase characteristic variable range) needed for the equalization amplifier, which cuts the cost of the equalization amplifier down and hence contributes greatly to decreasing the cost of this receiving apparatus.
Still furthermore, it is preferable that the aforesaid received waveform degradation detecting means is composed of received waveform measuring means for measuring waveform data (received waveform data) of the received signal and calculating means for calculating a difference between frequency data obtained by converting the received waveform data, measured by the received waveform measuring means, into a frequency domain and frequency data on the aforesaid reference waveform. This ensures acquisition of the difference in frequency data between the received signal and the reference waveform.
Incidentally, the aforesaid received waveform degradation detecting means acts as a received signal waveform degradation detecting device and the aforesaid received waveform measuring means serves as a received signal waveform measuring unit, with they being applicable to any type of receiving apparatus.
That is, a received signal waveform degradation detecting device according to the present invention measures waveform data (received waveform data) of a received signal received through a transmission line in a state subjected to waveform degradation (received waveform measuring process) and calculates a difference between frequency data on the received signal obtained by converting the received waveform data into a frequency domain and frequency data on the reference waveform free from the waveform degradation (calculating process), thereby detecting the degree of waveform degradation of the received signal; therefore, this enables the detection of the waveform degradation of a received signal in every receiving apparatus with high accuracy.
Moreover, the received signal waveform measuring unit according to the present invention equivalent-time-samples a received signal, received through a transmission line in a state where subjected to waveform degradation, to acquire a plurality of wave-height data on the received signal (equivalent-time sampling process) and records the acquired wave-height data as the waveform data on the received signal to be converted into a frequency domain for the calculation of the difference with respect to the frequency data on the reference waveform free from the waveform degradation (waveform data recording process); therefore, this ensures the measurement of the waveform data needed for the calculation in frequency data between the received signal and the reference waveform in any receiving apparatus.